This invention relates to high power laser applications in which the required optical output power is greater than can be obtained from a single laser or laser bar as a consequence of which multiple laser devices must be assembled in stacks.
Certain applications in the printing and welding industries, for example, require high intensity, high power laser beams. To provide the requisite power it is the practice to make an electrical connection of the assemblage of laser diode bars in the stack so that they are in series with each other. For example, certain high power diode pumped solid state laser applications may require up to 100 diodes to be connected in series in one cavity and up to 8 cavities may be needed. The typical current through a laser diode in a stack is 60A at a forward voltage of 1.8V. While such a stack assemblage permits the desired optical output power to be achieved, there is a problem should the heat incidentally generated in the stack cause any of the solder connections to melt. The resulting open circuit interrupts current to the laser stack and shuts down the laser""s optical output. The shut down of the optical output power is at least inconvenient to the user and also may have more serious consequences in ruining the product being operated upon.
U.S. Pat. No. 5,550,852 has heretofore shown that it can be advantageous to connect a reverse laser diode in parallel with each operative laser diode to provide a path for reverse currents which may occur during testing, power-up, etc., that otherwise would be damaging to the active laser diode. In the ""852 patent the reverse diode has electrical properties similar to those of the operative laser diode so that the reverse diode and the active diode could be fabricated at the same time. While the reverse diode is therefore useful, it does not solve the problem of maintaining operational continuity of a laser bar stack should one of the series connected laser diodes fail in an open-circuit condition.
In accordance with the principles of the present invention a stack serially connected laser diodes is prevented from failing in operational continuity when one of the laser diodes fails in an open circuit condition by proving an individual safety diode for each diode in the stack connected in parallel with its corresponding laser diode. The safety diode is fabricated so that it exhibits a higher forward voltage (higher band gap) than its corresponding laser diode. Should the laser diode fail in an open circuit condition, the voltage impressed across the open circuit will rise since it is no longer shunted by the low resistance of the failed laser diode. The higher voltage is sufficient to allow the safety diode to take over the function of the failed laser diode thereby maintaining continuity of the current path through the serially connected laser diodes of the stack. Advantageously, the safety diode is mounted on the same heat sink as its companion laser diode so that it can receive efficient cooling when it is called into operation. The light output from the safety diode when it turns on in response to the failure of its associated laser diode may advantageously be used to provide a different color light to signal the status of the assembly of multiple diode lasers.